Heroin with a street value of £40m has been found hidden under towels and bathrobes in a shipping container.
The haul, one of the biggest ever in the UK, was uncovered by the National Crime Agency (NCA) at Felixstowe dock in Suffolk on Friday.
Nearly 400kg (62st 13lb) of the drug was seized before the container was placed back on the ship and taken to Antwerp, Belgium.
Three people have been arrested in connection with the drug smuggling.
Two people were held on Monday when the container, which had been emptied of the drugs, was picked up by a lorry and driven to Rotterdam, Netherlands.
Simultaneously, a man from Bromsgrove, Worcestershire, was arrested.
The drugs would be worth at least £9m to organised criminals selling the whole consignment and at least £40m at street level in Europe, the NCA said.
Regional operations manager Colin Williams said: "The seizure of such a large quantity of heroin is the result of a targeted, intelligence-led investigation, carried out by the NCA with international and UK partners.
"It is almost certain that some of these drugs would have been sold in the UK, fuelling violence and exploitation including what we see in county lines offending nationwide."
Mark Kennedy, deputy director of Border Force, said: "Substantial seizures like this help to keep communities safe and hit the organised crime groups involved in the international drugs trade hard."